Which one of the following cement is best for the marine works?
š Challenges of Marine Environments
Marine structures are exposed to a highly aggressive environment. The primary threats to concrete are:
- Sulfate Attack: Sulfates present in seawater react with compounds in the cement paste (like tricalcium aluminate, CāA), leading to expansion, cracking, and deterioration.
- Chloride Attack: Chlorides from salt water penetrate the concrete and corrode the steel reinforcement, causing it to expand and spall the concrete cover.
Therefore, the best cement for marine works must have high resistance to both sulfate and chloride attacks.
š Detailed Analysis of the Options
(a) Blast furnace slag cement
This is an excellent choice for marine works and is widely used today. The slag content makes the concrete denser and less permeable, significantly improving its resistance to both chloride and sulfate attacks. While not the keyed answer for this specific question, it is arguably the most common and reliable choice in modern practice.
(b) High alumina cement (HAC)
This is the correct answer based on the provided exam key. HAC is manufactured from bauxite and limestone. It is characterized by its very high resistance to chemical attacks, especially from sulfates, due to the absence of free lime and low CāA content. It also gains strength very rapidly. Its high performance in chemically aggressive environments makes it a suitable choice for marine applications, as well as for industrial floors and refractory concretes (as per IS 6452:1989).
(c) Low heat Portland cement
This cement is designed to reduce the heat of hydration in mass concrete pours (like dams) to prevent thermal cracking. While it has moderate sulfate resistance, it is not the primary choice for the severe chemical exposure in marine environments.
(d) Rapid hardening cement
This cement is unsuitable for marine works. It achieves high early strength due to a higher CāA content, which makes it highly vulnerable to sulfate attack.
š Cement Suitability for Marine Works
| Cement Type | Key Feature | Suitability for Marine Works |
|---|---|---|
| Blast Furnace Slag Cement | Low permeability, high resistance to chlorides & sulfates. | Excellent. Often the preferred choice in modern construction. |
| High Alumina Cement | Very high chemical & sulfate resistance, rapid strength gain. | Very Good. Excellent chemical resistance but can have long-term stability issues ("conversion"). |
| Low Heat Cement | Low heat of hydration. | Moderate. Better than OPC, but not the best. |
| Rapid Hardening Cement | High early strength. | Poor. Highly susceptible to sulfate attack. |
š” Study Tips
- Marine = Chemical Attack: The main problem in marine works is chemical attack from sulfates and chlorides. Look for cements known for chemical resistance.
- HAC vs. Slag: Both High Alumina and Blast Furnace Slag cements are top contenders. For exam purposes, if HAC is an option, it's often keyed as the answer for "best" chemical resistance. In practice, slag cement is more common.
- Eliminate the Obvious: Rapid Hardening Cement is almost always the wrong answer for questions involving durability and chemical resistance.